Methods of making germanium alloy semiconductors



May 17, 1955 NORTH 2,708,646

METHODS OF MAKING GERMANIUM ALLOY SEMICONDUCTORS Filed May 9, 1951 TI'E-EIE! TEE-El]:

INVENTOR. HARPER Q. NORTH. BY

United States Patent METHODS OF NIAKING GERMANIUM ALLOY SEMICONDUCTORS Harper Q. North, Los Angeles, Calif., assignor, by mesne assignments, to Hughes Aircraft Company, a corporation of Delaware Application May 9, 1951, Serial No. 225,410

.12 Claims. (Cl. 1481.5)

The present invention relates to methods of making germanium alloy semiconductors and more particularly to methods of making germanium alloy semiconductors having both P-type and N-type regions with transition regions therebetween.

In the semiconductor art, a region of germanium con- A more detailed description of electrical conduction in N-type and P-type germanium may be found in the volume by Shockley entitled Electrons and Holes in Semiconductors, published in 1950 by D. Van Nostrand, Inc. of New York city. In addition, Shockley, on page 14, sets forth some typical donor impurities, such as antimony, phosphorus and arsenic, for example, and the more common acceptor impurities, such as boron, aluminum, gallium and indium, for example.

P-N junctions have numerous applications in the semiconductor art, among which are amplifiers, photosensitive devices, and diodes. Accordingly, an efficient method of making such junctions is of primary importance to the art. It generally has been considered that a true P-N junction could not be formed by simply placing two separate pieces of germanium of opposite conductivity in contact with each other, due to the lay-- ers of oxide or other material on thecommon surface,

to the surface states of the specimens, to thoroughness of the specimens, and to other conditions of the contact.

P-N junctions, therefore, have been formed from a continuous solid specimen by artificial methods.

One prior art'method of forming -P-N junctions is bynucleon bombardment 'or irradiation. According to this specimen "being shielded by lead. If the bombardment is continued for a sufiicicntly long period, the resulting en'd' product will be P-type. Accordingly, it the original specimen "were" N-type'," the P'-N junction. H H i This method of forming P-N junctions has several disadvantages, the primary one of which is that an atomic file or high energy acceleration is required to produce the nucleon bombardment. In addition, the starting material must always be N-type, since known nuclear reactions have always produced a P-type end product. Furthermore, the bombardment of the germanium specimen does not permit control of the distribution of the excess holes, and it is therefore difi'icult to control the production of the P-type region and the extent of the transition region.

Another prior art method of forming P-N junction employs a heated germanium specimen of N-type which resulting product will be a ample s.

is partially quenched in a liquid. The resulting alteration of the lattice structure of the portion of the specimen quenched in the liquid produces a P-type region of this portion. However, this P-type region exists only temporarily and eventually reverts back to its original -type, particularly under the annealing action of elevated temperatures.

The present invention discloses methods of making P-N junctions in a germanium specimen which obviate the above and other disadvantages of the prior art. According to one method of the present invention, an active impurity, that is either a donor or acceptor impurity, is alloyed or mixed with a portion of a germanium specimen of either P-type or N-type so as to produce a P-N junction at the boundary between the remainder of the specimen and the portion which has alloyed with the impurity. The particular active impurity utilized, that is whether it is a donor or an acceptor, depends upon the type of the germanium specimen. By controlling the amount of impurity introduced and the location and rate of its introduction, an excellent control can be exercised over the extent of the transition region and the rectification ratios produced. Furthermore, by subtequent treatment of the resulting specimen, such as etching of surfaces where contacts are made and annealing of the specimens, the rectification ratio may be increased.

According to a modified method of the present invention, impurities of both types are added to a germanium specimen in such a manner as to produce a germanium alloy semiconductor having varying concentrations ofimpurities thereacross. In this manner. the resulting semiconductor may more nearly approach an ideal asymmetrical semiconductor as set forth below.

Accordingly, it is an object of the present invention to provide a method of making germanium alloy semiconductors having both P-type and N-type regions.

Another object of the invention is to provide a method of making a P-N junction from a germanium specimen.

A further object is to provide a method of adding active impurities to a germanium specimen to form a P-N junction.

Still another object is to provide a method of melting a germanium specimen and adding active impurities thereto to form a P-N junction.

A still further object of the present invention is to provide a method of making a germanium alloy semiconductor having regions of'varying concentrations of impurities thereacross.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with' the accompanying drawings in which several embodiments of the invention are illustrated by way of exthe limits of the invention.

Fig. 1 is a'schematic diagram, partly in section, of

one form of apparatus for performing the method of,

the present invention;

Fig. 1a is a view similar to Fig. 1, illustrating the apparatus in impurity adding position;

formed when the starting material is relatively pure or intrinsic germanium;

Figs. 4a and 4b are schematic diagrams, partly in Patented May 17, 1955 It is to be expressly understood, however, that the drawings are for the purpose of illustration and de scription only, and, are not intended as a definition of section, of the apparatus of Fig. l for performing a modified method of the present invention;

Figs. 5a and 5b are longitudinal sectional views of the product resulting from the methods illustrated in Figs. 4a and 4b, respectively; and

Fig. 6 is a schematic diagram partly in section of another form of apparatus for performing the modified method of the present invention.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Fig. 1 one form of apparatus for carrying out the method of the present invention. The apparatus comprises a crucible 11 for receiving the germanium to be treated, crucible i1 preferably being made of a high melting point conductive material which does not react with the germanium, such as graphite. Crucible 11 has a pair of central longitudinal connecting bores 12 and 13, bore 12 having a larger diameter and a shorter length than bore 13, and being positioned adjacent the upper end of crucible 11. An induction or radiant heating coil 14 surrounds crucible 11. and is coaxial therewith, coil 14 being connected to a suitable source of energy 15. Crucible 11 is movable with respect to coil 15 in the directions indicated by arrows 16.

In operation, a wafer 17 of germanium is placed in bore 12, and the energy from source 15 is applied to coil 14 so that the heat energy radiated by coil 14 or the energy transferred to the crucible in the case of induction heating melts wafer 17 and causes the germanium to flow into and substantially completely to fill bore 13. Although it is clear that wafer 17 may be melted in bore 12 without entering bore l3, it is preferable to provide a narrow bore of the configuration of bore 13, since the long narrow ingot 18 thus formed is more readily susceptible to further handling and to use as a PN junction.

Although, as set forth below, wafer 17 may be composed of pure germanium or germanium of either P-type or N-type, it is assumed for the present that wafer 17 is P-type. After ingot 13 has solidified, a portion thereof adjacent the upper end is cut off by any suitable means and a doner impurity 19 is inserted between the main body portion of ingot 13 and the cut portion, as shown in Fig. la. Crucible 11 is then lowered relative to coil 14 to the position shown in Fig. la. In this position, only the top half of ingot t3 will melt when energy is applied to coil 14.

It should be pointed out that ingot 18 is cut in the manner described primarily to provide mechanical holding of impurity 19 by sandwiching it between the two portions of ingot 18. In addition, certain of the donor impurities, such as antimony and arsenic, have relatively high evaporation rates and these impurities might at least partially evaporate before ingot 13 melted. Accordingly, the sandwiching of impurity 19 minimizes loss due to evaporation. It is, to be understood, however, that for certain donor impurities, the cutting of ingot 18 may be eliminated, and impurity 19 may be positioned directly upon the top surface of ingot 18.

The amount and type of donor impurity 19 added to ingot 1S depends upon the desired. characteristics of the end product. Generally, it may be stated that germanium having a low impurity concentration exhibits a high peak inverse voltage and a low forward current, while high impurity concentration germanium has a low peak inverse voltage and a high forward current. Accordingly, the amount of impurity 19 added will vary, with a suitable value being of the order of a fraction of an atomic percent of the germanium to which the impurity is to be added. In practice, antimony of an amount of the order of one-hundredth of an atomic percent has proved satisfactory.

With the apparatus in the position shown in Fig. la, energy is applied to coil 14 to heat impurity 19 and the top half of ingot 18. As ingot 18 melts, impurity 19 spreads throughout the upper half of ingot 18 and produces N-type germanium in this upper molten half. Depending upon the type of impurity used, impurity 19 may dissolve and spread throughout the molten portion of ingot 18 adjacent, or may alloy or mix with the molten portion of ingot 18 before the ingot reaches its melting point. In either case, the adjacent portion of ingot 18 is converted from a solid state to a liquid state and this conversion is termed melting within the meaning of this application. As the lower half of ingot 13 remains solid and P-type, a PN junction is formed as shown in Fig. 2. Suitable probe and microscopic tests have confirmed the conclusion that a P\ junction is present, and that the transition region 21 between the P-type and N-type regions is sharp and acts as a rectifying barrier, that is the current that flowed from the P-type region to the N-type region for a given voltage far exceeded the current that flowed in the opposite direction for the same voltage.

Although the method thus far described produces a P-N junction, additional steps may be taken to increase the rectification ratio of the junction. Thus, annealing of the ingot, or of a portion thereof, will improve the rectification ratio, one of the reasons being that annealing Will remove lattice distortions in the ingot caused by cooling of the same. A central longitudinal section was cut out of ingot 18 and annealed at 550 C. for a. period of three hours. Rectification tests before and after annealing indicated a great improvement in the rectification ratio by annealing.

In addition, it has been ascertained that impurities, other than the donor impurity, form on the surface of ingot 18 during the formation of the P-N junction. By performing all of the operations in an inert atmosphere, such as helium, or in a vacuum, the possibility of oxide formations on the surface of ingot 13 is removed. Furthermore, etching of the ingot, or portion thereof, will effectively remove any other impurities. Tuus, etch ing of the central section with a mixture of hydrofluoric acid and nitric acid further increased the rectification ratio of the section.

Although the method has been described in connection with a P-type germanium starting material, it is clear that the starting material may he N-type or relatively pure germanium. Thus, if N-type germanium is used, the method remains the same, except that impurity 19 is of the acceptor type rather than the donor type. Any suitable acceptor impurity, that is any impurity which causes a deficit of free electrons or a surplus of holes in the starting material, may be used.

Where the starting material is intrinsic germanium, that is neither N-type or P-type for all practical purposes, a procedure identical with that set forth above is followed. This procedure will produce an ingot, generally designated 31 in Fig. 3, which comprises a first section 32 of N-type and a second section 33 of intrinsic germanium. By reinserting ingot 31 in crucible 11 so that section 32 is lowermost, and by again following the steps of, the method with an acceptor impurity, section 33 is converted into a P- type region and a PN junction is formed in ingot 31. Obviously, the peak inverse voltage and the conductivity of a P-N junction made in this manner will be determined almost entirely by the impurity concentration.

As pointed out above, the peak inverse voltage of the P-N junction is inversely related to the concentration of impurities, while the conductivity of the junction increases with net concentration gradient of impurities. Since the ideal junction is one having a high peak inverse voltage and a high conductivity, it becomes desirable to produce an ingot having varying concentrations of impurities thereacross in order to approach the ideal junction. This result. may be. attained. by the method of the present invention.

Referring now to Fig. 4a, the apparatus of Fig. l is shown as being loaded with an ingot 41 of the N-type germanium. ingot 41 is of relatively high purity, that is even though ingot 41 is N-type it contains a relatively small impurity concentration. As shown in Fig. 4a, crucible 11 is positioned relative to coil 14 so that less than the upper half of ingot 41 is melted by the heat generated by coil 14. To produce a P-type region, an acceptor impurity 39 is sandwiched between a cut-off section and the remaining section of ingot 41.

Upon applying energy to coil 14, the upper portion of ingot 41 is melted, and impurity 39 spreads throughout this melted portion. The resulting ingot 43, as shown in Fig. 5a, comprises an N-type region of relatively high purity, a P-type region of relatively low purity, and a transition region or barrier 44. The relative purities of the regions are indicated by the densities of the shading lines, the greater the density of the lines the higher the impurity concentration.

Ingot 43 is then inserted in crucible 11 with the P- type region lowermost, as indicated in Fig. 4b, the top end of the N-type region of ingot 43 being cut otf so that a donor impurity 49 may be inserted between the portions of the Nrtype region. With crucible 11 positioned relative to coil 14 as shown in Fig. 4b, that is less than the upper half of ingot 43 being in position tobe melted by coil 14, energy is applied to coil 14 to melt the upper portion of ingot 43. Impurity 49 spreads throughout the melted portion of ingot 43, and the resulting ingot 45 is shown in Fig. 5b. Ingot 45 comprises a P-type region of relatively low purity, an N-type region of relatively low purity, an intermediate N-type region of relatively high purity, and a transition region or barrier 46 between the P-type region and the intermediate N-type region.

It is thus seen that ingot 45 possesses regions of different impurity concentrations, with the end regions exhibiting relatively high conductivity. Obviously, by varying the extent of each of the regions and by varying the amounts of donor and acceptor impurities, the characteristics of the resulting junctions may be varied to suit the purposeat hand. Furthermore, it is clear that the starting materialmay be extrinsic germanium of either type, that is germanium containing an excess of either donor or acceptor impurities.

In each of the embodiments of the present invention thus far described, crucible 11 has been composed of conductive material. Where localized heating and melting is desired, an insulating crucible with conductive shields may be utilized to melt simultaneously more than one region of the ingot. One typical embodiment of this arrangement is illustrated in Fig. 6. 7

Referring now to Fig. 6, a crucible, generally designated 61, composed of suitable electrical insulating material such as quartz, is provided with a pair of communicating bores 62 and 63 identical with bores 12 and 13 of Fig. l. Crucible 61 is positioned coaxially with respect to an induction coil 14 which is connected to a suitable source of electrical energy. Positioned between coil 14 and crucible 61 and coaxial therewith are a pair of conducting rings 64 and 65, rings 64 and 65 being arranged to radiate and concentrate energy coupled from coil 14 in the upper and lower ends, respectively, of bore 63.

In operation, an ingot 66 of germanium is inserted into bore 63, ingot 66 being either relatively pure germanium or low impurity germanium of either N-type or P-type. In the arrangement shown in Fig. 6, the upper and lower ends of ingot 66 are cut from the main body and impurities 67 and 68, respectively, are sandwiched between the ends of themain body. It should be understood, however, that the method of the present invention may be performed by inserting impurities 67 and 68 above and below ingot 66, respectively, in bore 63, without cutting ingot 66.

With the apparatus and material in the position shown in Fig. 6, energy is applied from source 15 to coil 14 and is thereupon coupled to the rings to heat impurities 67 and 68, and to heat and melt the portions of ingot 66 aligned with rings 64 and 65. In this manner, the end portions of ingot 66 melt, while the central portion of ingot 66 remains solid. Upon the melting of ingot 66, impurities 67 and 68-spread throughout their respective portions, and, if one impurity is a donor and the other is an acceptor, produce N-type and P-type regions in ingot 66. Thus a multiple junction of varying impurity concentration is formed, the characteristics of which are determined by the type of starting material for ingot 66, the amount of impurities 67 and 68 added, and the extent of the regions of ingot 66 melted by coil 14 and rings 64 and 65.

In Fig. 6, if the starting material is either P-type or N-type, a final product of either NPN type or PNP type may be produced. Thus, if P-type starting material is used, as shown in Fig. 6, impurities 67 and 68 would be donor impurities and the product produced by the apparatus of Fig. 6 would be NPN type. Similarly, with N-type starting material and acceptor impurities the resulting product is PNP.

It is thus seen that the present invention provides a method of making germanium alloy semiconductors having both P-type and N-type regions by the addition of impurities to a partially molten germanium specimen. The extent of the transition regions and rectification ratios of the resulting product may be controlled by the amount of impurities added and the location and rate of introduction of these impurities. Subsequent etching and annealing of the resulting product serves to increase the rectification ratio and to remove extraneous impurities. I What is claimed as new is:

l. The method of making a germanium alloy semiconductor having both P-type and N-type regions by the addition of an acceptor impurity to a specimen of N-type germanium, said method comprising the steps of melting a portion of the specimen, spreading the impurity through the molten portion of the specimen, and cooling the combination to solidify the molten portion of the specimen, together with the acceptor impurity, as a P-type region in rectifying relationship with the remainder of the speci men.

2. The method of making a germanium alloy semiconductor having both P-type and N-type regions by the addition of a donor impurity to a specimen of P-type germanium, said methodcomprising the steps of partially melting the specimen, simultaneously melting the donor impurity, spreading the molten impurity through the molten portion of the specimen, and cooling the molten portion of the specimen, together with the donor impurity,

to create a donor-impurity-doped N-type region in the specimen.

3. The method of making a germanium alloy semiconductor having both P-type and N-type regions by the addition of a donor impurity to a specimen of P-type germanium, said method comprising the steps of positioning the donor impurity in direct proximity with one region of the specimen, melting said one region of the specimen and said impurity to permit said impurity to spread throughout said one region of the specimen and create a molten alloy region, and cooling the molten alloy region to produce a donor-impurity-doped N-type region in the specimen.

4. In the method of making a germanium alloy semiconductor having both P-type and N-type regions by the addition of at least one acceptor impurity to a specimen of N-type germanium, the combination of steps comprising cutting the specimen into two sections, sandwiching the acceptor impurity between the two sections of the specimen, and simultaneously heating the impurity, and melting one of the sections of the specimen and a portion of the other section of the specimen adjacent the impurity, thereby permitting the impurity to spread through the one molten section and the molten portion of the other section of the specimen.

5. The method of making a germanium alloy semiconductor having regions of varying concentrations of active impurities from a specimen of extrinsic germanium containing an excess of active impurities of one type by the addition to the specimen of an active impurity of the donor type and an active impurity of the acceptor type, said method comprising the steps of melting two spaced regions of the specimen, heating the impurities to be added, and spreading the heated impurities through the two molten regions, respectively, of the specimen.

6. The method of making a P-N junction from a germanium specimen containing an excess of donor impurities by the addition to the specimen of an acceptor impurity, said method comprising the steps of melting the acceptor impurity and partially melting the specimen, spreading the molten impurity through the molten region of the specimen, and cooling the molten region of the specimen, together with the acceptor impurity d3 crsed therein, to solidify the molten region as an eprcrimpurity-doped P-type region.

7. The method of making a germanium alloy semiconductor having regions of varying concentrations of active impurities from a specimen of extrinsic germanium containing an excess of active impurities of one type by the addition to the specimen of an active impurity of the donor type and an active impurity of the acceptor type, said method comprising the steps of melting a first region of the specimen, heating one of the impurities to be added, spreading the one heated impurity through the one molten region of the specimen, cooling the specimen to permit the specimen to solidify, melting another region of the specimen spaced from the one region, heating the other of the impurities to be added, and spreading the other heated impurity through the other molten region of the specimen.

8. The method of making a germanium alloy semiconductor having regions of varying concentrations of active impurities from a specimen of extrinsic germanium containing an excess of active impurities of one type by the addition to the specimen of active impurities of the other type, said method comprising the steps of melting two spaced regions of the specimen, heating the impurities to be added, and spreading the heated impurities through the two molten regions, respectively, of the specimen.

9. The method of making a germanium alloy semiconductor having both P-type and N-type regions by the addition of a donor impurity to a specimen of P-type germanium, said method comprising the steps of melting a portion of the specimen, spreading the impurity through the molten portion of the specimen, and cooling the corn bination to solidify the molten portion of the specimen, together with the donor impurity, as an N-type region in rectifying relationship with the remainder of the specimen.

10. The method of making a germanium alloy semiconductor having. both P-type and N-type regions by the addition of an acceptor impurity to a specimen of N-type germanium, the combination of steps comprising positioning the acceptor impurity in direct proximity with one region of the specimen, melting said one region of the specimen and said impurity to permit said impurity to spread. throughout said one region of the specimen and create a molten alloy region, and cooling the molten alloy region to produce an acceptor-impurity-doped P-type region in the specimen.

11. The method of making a P-N junction from a germanium specimen containing an excess of acceptor impurities by the addition to the specimen of a donor impurity, said method comprising the steps of melting the donor impurity and partially melting the specimen, Spreading the molten impurity through the molten region of the specimen, and cooling the molten region of the specimen, together with the donor impurity dispersed therein, to solidify the molten region as a donor-impuritydoped N-type region.

12. The method of making an N-P-N germanium alloy semiconductor having regions of varying concentrations of active impurities from a specimen of extrinsic P-type germanium containing an excess of acceptor impurities by the addition to the specimen of donor impurities, said method comprising the steps of melting two spaced regions of the specimen, melting the impurities to be added, and spreading the molten impurities through the two molten regions, respectively, of the specimen.

References Cited in the file of this patent UNITED STATES PATENTS 1,565,496 Pfeil Dec. 15, 1925 2,402,582 Scafl June 25, 1946 2,402,839 0111 June 25, 1946 2,447,829 Whaley Aug. 24, 1948 2,504,628 Benzer Apr. 18, 1950 2,505,633 Whaley Apr. 25, 1950 2,514,879 Lark-Horovitz et al. July it, 1950 FOREIGN PATENTS 632,942 Great Britain Dec. 5, 1949 

1. THE METHOD OF MAKING A GERMANIUM ALLOY SEMICONDUCTOR HAVING BOTH P-TYPE AND REGIONS BY THE ADDITION OF AN ACCEPTOR IMPURITY TO A SPECIMEN OF N-TYPE GERMANIUM, SAID METHOD COMPRISING THE STEPS OF MELTING A PORTION OF THE SPECIMEN, SPREADING THE INPURITY THROUGH THE MOLTEN PORTION OF THE SPECIMEN, AN COOLING THE COMBINATION TO SOLIDIFY THE MOLTEN PORTION OF THE SPECIMEN, TOGETHER WITH THE ACCEPTOR IMPURITY, AS A P-TYPE REGION IN RECTIFYING RELATIONSHIP WITH THE REMAINDER OF THE SPECIMEN. 